Nowadays, line frequency AC/DC power supply has been widely replaced by switching mode AC/DC converter. For a constant frequency pulse width modulation (PWM) AC/DC converter, it generally operates at 67 KHz or about 100 KHz. As is known, most of time an AC/DC converter is in light load or no load condition, such as standby mode or battery removed in charger. During such condition, output power is much reduced but power loss will not. This will lead to poor efficiency. Regulations have been made to limit power consumption under such condition. Under light load condition, switching loss dominates the overall power loss. The key to increase efficiency is to reduce switching frequency under light load. There are many arts related to light load efficiency improvement, for example U.S. Pat. No. 6,212,079 to Balakrishnan et al., U.S. Pat. No. 6,252,783 to Huh et al., and U.S. Pat. Publication No. 20030117813 of Hong et al. Methods to increase efficiency can be categorized into three groups: pulse skipping mode, off time modulation mode, and cycle skipping or burst mode.
FIG. 1 shows a flyback converter 100 with its primary controller 102. The converter 100 has a transformer TX and a power switch SW serially connected to a primary winding Lp of the transformer TX, and the controller 102 provides a switching signal S1 by its pin GATE to switch the power switch SW for power delivery to a secondary winding Ls of the transformer TX. The transformer TX further has an auxiliary loop Laux to provide a current Iaux to charge a capacitor Cvcc for providing a supply voltage Vcc to the controller 102, and an optocoupler 104 detects an output voltage Vout of the converter 100 to feed back a signal Vfb to a pin FB of the controller 102. The optocoupler 104 includes a transistor 106 optically coupled with a light emitting diode (LED) D2, and a compensation network 108 to compensate the input signal of the optocoupler 104. Besides operating the converter 100 in fixed frequency PWM and providing protection functions, the controller 102 has to take care of light load efficiency to meet regulation requirement.
For pulse skipping mode, as load decreasing, some pulses from the oscillator are skipped. In FIG. 2, waveforms 200 and 202 represent the switching signal S1 provided by the controller 102 at full load and light load, respectively. As load RL decreasing, the controller 102 enters pulse skipping mode and, as shown by the waveform 202, some pulses 2022 are skipped. However, the oscillator in the controller 102 still generates constant frequency clock as in normal load. As the number of the pulses in the switching signal S1 reduces, the switching times of the power switch SW consequently reduces. Skipped pulses can be limited to prevent the system from operating into audio frequency. The ripple control under light load is easy to achieve.
For off time modulation mode, as load decreases, off time is increased so the switching frequency is also reduced. The lighter the load, the longer the off time. In FIG. 3, waveforms 204 and 206 represent the switching signal S1 provided by the controller 102 at full load and light load, respectively. As shown by the waveform 206, as load RL decreases, the controller 102 increases the off time Toff of the switching signal S1. Generally, this method is applied in “quasi-resonant converter”. In pulse skipping mode and off time modulation mode, the frequency can be continuously reduced.
For burst mode, due to isolation, feedback voltage Vfb instead of output voltage Vout is taken to judge loading and compare with upper limit and lower limit of burst mode control. In FIG. 4, waveforms 208 and 212 represent the switching signal S1 provided by the controller 102 at full load and light load, respectively, and waveform 210 represents the feedback signal Vfb. As the feedback signal Vfb reaches upper limit V_high, the oscillator clock and thereby the switching signal S1 are blanked. As the feedback signal Vfb reaches lower limit V_low, the oscillator clock and thereby the switching signal S1 are released. The output ripple can be controlled by adjusting the upper limit V_high and lower limit V_low. However, the burst mode may cause the controller 102 turning off abnormally when loading becomes further lighter or no load.
Therefore, it is desired a method and apparatus to improve the efficiency when a switching mode converter operates at much lighter load or no load.